This invention relates to a method for preparing porous metallic-matrix hydride compacts. More specifically, this invention relates to a method for preparing porous metallic-matrix hydride compacts which are able to withstand repeated hydriding-dehydriding cycling without disintegrating.
The utilization of hydrogen, an ideal, nonpolluting fuel, as an alternative to fossil fuels is attracting much attention. Hydrogen has been suggested as a working fluid in a closed system utilizing thermal energy from low grade heat sources to provide industrial and residential space heating. Hydrogen is also being considered for use in vehicle propulsion and in electric peak shaving systems such as fuel cells for producing electricity during peak periods of demand. The use of hydrogen as a chemical heat pump for applications in refrigeration and for upgrading low-quality heat energy is also presently under investigation however, almost any method under consideration requires a safe and effective means for the storage of the hydrogen.
The use of metal hydrides produced in a reversible chemical reaction with hydrogen provides an excellent solution to the hydrogen storage problem. Heat must be removed and supplied in order that the reactions can proceed. Hydrogen storage units consisting of sealed containers filled with a metal hydride bed and subsystems for heating, cooling and pressure control have been constructed and utilized. However, the heat transfer rate is of major concern for the effective utilization of such systems.
Thus far, only metal hydrides in the form of powder have been considered for hydrogen storage, and hydrides as powders have a very low thermal conductivity. The poor heat transfer capabilities of a powder metal hydride bed, however, is a considerable restriction on the design and construction of hydride storage systems. The metal hydride powders are generally of a fine particle size which makes it necessary to use filters to prevent the particles from being entrained in the gas stream. Furthermore, the repeated cycling causes the fine particle size to become even further reduced, causing filter congestion and increasing the pressure drop throughout the hydride bed. In most of these applications the heat transfer rate controls the hydrogen flow, so that a high hydrogen flow rate requires a high heat transfer rate. Thus complicated high-surface-area heat exchangers must be used if fast cycling is required.
Attempts have been made to improve the heat transfer capabilities of the metal hydride beds. For example, the hydrides have been placed in containers made of a highly porous metal. However, it has proved difficult to properly seal the containers of the porous metal to prevent loss of hydrogen. Other complicated heat exchanger configurations placed within a bed of the powdered metal hydrides have been tried but none have proven to be totally successful.
Hydrides compacted into porous solids supported by a thin metal matrix which does not absorb hydrogen have also been suggested. Furthermore, it has been calculated that these porous metal hydrides would show greatly improved thermal conductivity and diffusivity. The preparation of such porous metal hydrides has been tried using materials such as aluminum, nickel and copper as the binding metal matrix, by such methods as liquid-phase sintering, solidstate sintering or high-pressure compaction at room temperature. As it has turned out however, none of these methods produced a compact that was sufficiently strong to withstand the stresses arising from volume increases resulting from the formation of the metal hydrides. While absorbing hydrogen, each hydride particle imposes a compressive stress on its nearest neighbor hydride particles, which builds up to very high levels at a distance of a few coordination spheres. The binding material is not able to withstand these stresses and compacts made in this manner begin to disintegrate within one or two hydriding-dehydriding cycles, powdering the hydride and losing the enhanced heat transfer capabilities available in the porous compact.